Ouch, that sounds bad. I suggest taking the penny out and doing an immediate water change until someone else can help. Seeing as you don't know where the penny has been, you don't know what could be on it, so scrub the tank down very well.

I scrubbed the pennies really well before dropping them in, but he turned back to being a dark cobalt blue. Someone asked me if he was in complete darkness before i noticed the sudden color change and I said yeah. THey told me betta's get pretty pale when they're sleeping, i never knew that lol now i feel silly for worrying so much